1. Field of the Invention
This invention relates to improvements in a filter for trapping impurities or particles contained in a fluid such as oil, more particularly to the improvements in a filter adapted to filter the fluid to flow through a fluid flow passage extending through inside and outside of a cylindrical member.
2. Description of the Prior Art
A filter for filtering oil flowing through an oil flow passage extending the inside and outside of a plug forming part of a pressure relief valve is disclosed in Japanese Utility Model Provisional Publication No. 5-96477, in which oil is released through the relief valve disposed in the oil flow passage from a vane pump in a power steering system of an automotive vehicle for the purpose of pressure relief. Such a filter 20 is shown in FIGS. 4 and 5: A wire gauze is formed generally cylindrical and provided at its opposite ends with inner flange sections 21A, 21A, which are bent, and then annular retainers 22, 22 are caulked and installed respectively to the inner flange sections 21A, 21A. This filter 20 is used for fitting on the cylindrical section 30A' of a plug. The which cylindrical section 30A' is formed with an oil passage 30B' forming part of the oil flow passage through which oil is released for the pressure relief purpose. The filter 20 is located to cover the inlet side of the oil passage 30B' to trap impurities or particles contained in oil flowing through this oil passage 30B'.
However, difficulties have been encountered in the above-discussed conventional filter 20, in which the filter 20 is complicated in structure and difficult in production because of requiring caulking so that it unavoidably becomes large-sized, requiring a high production cost. Additionally, it is required to prepare many filters having different inner diameters, for the plugs 30' having different sizes of the cylindrical sections 30A'. Furthermore, when the filter 20 is fitted on the cylindrical section 30A', it is required to align the both axes of the filter 20 and the cylindrical section 30A' thereby making troublesome the operation of fitting the filter on the cylindrical section 30A'.